The input to output voltage conversion ratio of a current mode controlled power converter will affect the duty cycle that is produced by the control circuit of the current mode controlled power converter. When the duty cycle is larger than 50%, sub-harmonic may happen and cause the power converter seriously unstable and thereby the connected load abnormally operative. For current mode controlled power converters, a popular solution to eliminate sub-harmonic is to add a ramp signal into the modulator for generating modulation signals.
For example, as shown in FIG. 1, a current mode controlled buck power converter includes a buck switching regulator 10 for converting an input voltage Vin into an output voltage Vo, and a control circuit 12 for providing control signals Sq1 and Sq2 to drive the buck switching regulator 10. The control circuit 12 includes a current sense gain circuit 16 for detecting the inductor current IL of an inductor L in the buck switching regulator 10 to generate a current sense signal CS, a ramp generator 20 for providing a ramp signal Vramp for eliminating sub-harmonic, a feedback circuit 26 having two serially connected resistors Rd1 and Rd2 for dividing the output voltage Vo to generate a feedback signal Vfb, a compensation circuit 24 for amplifying the difference between the feedback signal Vfb and a reference voltage Vref1 to generate an error signal Vcomp, a modulator 17 for generating a modulation signal Sc according to the signals CS, Vramp and Vcomp, and a driver 14 for generating the control signals Sq1 and Sq2 responsive to the modulation signal Sc. The modulator 17 includes an adder 18 for adding the current sense signal CS and the ramp signal Vramp to generate a modified current sense signal CS′, and a comparator 22 for comparing the current sense signal CS′ with the error signal Vcomp to generate the modulation signal Sc. However, the existing power converters are made suitable for wide range input voltage and wide range output voltage, and therefore, if a constant ramp signal Vramp is used to eliminate sub-harmonic, then users or designers have to adapt the inductor L to different conditions of the input voltage and the output voltage, otherwise sub-harmonic will still happen in certain conditions of the input voltage and output voltage. This is an inconvenience to users and designers. In addition, any change to the inductor L requires corresponding change to the compensation design of the entire system loop, or the system loop would have its stability impaired.
For adaptive applications of different output voltages Vo, the existing current mode controlled power converter always provides a recommended component selection for users to select suitable component parameters depending on the application condition to ensure the system circuit to have stable power output under this condition. As the inductor L and the compensation parameters have to be altered for different conditions of the input voltage Vin and the output voltage Vo, the existing current mode controlled power converters always have more complicated recommended component selections.